The present invention relates to a microwave induced plasma (MIP) torches for use an ion source for mass spectometry.
Information on trace elements is important from both nutritional and toxicological standpoints. The naturally occurring levels of trace elements in our environment are continually disturbed by today's lifestyle. The products we manufacture and consume and the fuels we use make a contribution toward shifting these natural levels. One of the roles of analytical laboratories is to develop and apply methods to determine the nature and quantity of trace elements in the environment and in our food supply.
The analyst has a wide variety of instrumental techniques (e.g., electrochemical, nuclear, spectroscopic) available which are capable of performing elemental analysis at trace levels. However, all have specific limitations such as the number of elements to which the technique is applicable, the type of sample matrix which can be handled by a particular technique, the detection limits which can be achieved, the amount of sample required for the analysis, and the time required to perform the analysis. Each of these factors plays a role in determining which technique is most appropriate for the analysis at hand.
During the past decade, the field of atomic spectrometry, which includes the most widely used techniques for elemental analysis, has undergone a shift in emphasis from the use of flames as excitation sources to the use of plasmas sustained by electromagnetic fields. The most popular analytical plasma is the radio frequency inductively coupled plasma (ICP). This plasma is widely used for generating excited state atoms and ions from samples introduced to the plasma in optical emission work and has recently generated tremendous interest as an ion source for mass spectrometry Gray, 1975; Houk, et al., 1980.
Interest in this powerful new analytical technique has steadily increased since the first report of ion sampling directly from atmospheric pressure plasma into a mass spectrometer. Prior to the development of plasma source mass spectrometry, no single instrumental technique had been potentially capable of providing sub part-per-billion detection limits for the entire periodic table at nearly simultaneous detection speeds. Using plasma mass spectrometry, elemental isotopes are separated on the basis of mass to charge ratio. Samples are introduced into the plasma source as gases or liquids without physically placing the sample inside the vacuum chamber, which is typical of conventional mass spectrometric ion sources. A schematic of an atmospheric pressure plasma sampling mass spectrometer which was built in these laboratories is shown on FIG. 6 and illustrates a sampler (a), a skimmer (b), a first vacuum stage and pumping ports (c), ion optics (d), viewing port (e), second vacuum stage diffusion pump (f), quadrupole mass analyzer (g), thrid vacuum stage turbomolecular pump (h), analog amplifier (i), and pulse amplifier (j).
Initial plasma source studies used a dc capillary arc plasma which had major drawbacks due to a low plasma gas temperature, resulting in a total ion population which contained a large percentage of low ionization potential molecular oxides Gary, 1975. The analyte response depended on the ionization potential of the element with respect to the ionization potential of the molecular oxide. As the analyte ionization potential approached that NO (ca. 9.25 eV), which was the principle ion in the mass spectrum, analyte response degraded. The NO.sup.+ originated from air entrained in the plasma. The analyte response was also suppressed by easily ionized species such as sodium which originated from the sample. This was partially due to an inability to confine the sample to the "hot" region of the plasma. These problems rendered the CAP-MS impractical for "real" sample analyses where analyte generally exists in a complex matrix. Plasmas currently under investigation as ion sources are those whose principle applications have been in optical emission work, specifically the rf inductively coupled plasma mentioned above and the microwave induced plasma.
The inductively coupled plasma (ICP) has an analyte rich central channel produced by the introduction of aqueous aerosol. This central channel has a high analyte ion density (as demonstrated by optical emission techniques) and a high gas temperature associated with this channel provides the ICP with relative freedom from matrix interferences such as ionization suppression. The relatively simple background spectrum associated with ICP-MS is produced by the ionization of the plasma gas and its impurities along with polyatomic ions produced by plasma gas atoms combining with plasma gas, oxygen, or matrix elements Tan and Horlick, 1986. Examples of atomic argon and argon containing molecular ions which interfere in elemental determinations are listed in Table 1. Major interferences are at m/z=39, 40, 41, 54, 56, 80. Materials used in the construction of the interface are also evident in the background spectra. Background intensities observed are also dependent on instrumental parameters such as plasma sampling depth, nebulizer gas flow, solution introduction rate, Rf power, and electrostatics ion lens potentials. Experiments have demonstrated that analyte signal is influenced (i.e., suppression or enhancement of analyte signal) by sample matrices which contain high levels (above 0.1 %) of other ionizable elements. These matrix effects are a consideration in "real" sample analysis Satzger, 1988. The limitations and capabilities of ICP-MS in dealing with sample matrices are currently being investigated. Although matrix effects associated with ICP-MS are more pronounced than with ICP-AES, the excellent sensitivity for most elements, the multi-element capabilities, and the large linear dynamic ranges have made ICP-MS a promising technique for ultra trace elemental and isotopic analyses.
Microwave induced plasma (MIP) have been employed as atom or ion reservoirs in various analytical applications including optical emission and mass spectrometry Douglas and French, 1981; Satzger, et al., 1987; Satzger, et al., 1988. The MIP has received its only commercial acceptance as an atom specific detector for gas chromatography. In this application, analyte is separated from solvent and matrix components of the sample prior to introduction into the plasma. Therefore, the MIP, which is thermally cooler than the inductively coupled plasma, can more easily accomplish the tasks of atomization, excitation, and ionization.
The reasons for a lack of greater interest in the MIP among analytical laboratories are the difficulties which the MIP experiences in handling complex matrices and the difficulty the operator experiences in maintaining the plasma. The matrix problem is exemplified by a change in the magnitude and/or stability of analyte response in comparison to the response for an elemental standard during the time period in which the analytical measurement is made Satzger, Fricke, Caruso, 1988. Maintaining the plasma includes initiation, tuning, and sustaining a stable plasma which does not intersect the walls of the plasma containment tube (typically quartz) resulting in tube degradation. A stable plasma exhibits an efficient transfer of power from the microwave generator to the resonant cavity and is dependent on an impedance match between the generator and, in this case, a TM.sub.010 resonant cavity. A poor impedance match results in current reflected back toward the microwave power supply causing heating of connectors, cables, and tuning components as well as reduced magnetron lifetime.
The microwave induced plasma used as anion source for mass spectrometry offers several potential advantages over the rf Ar ICP. The microwave cavity can readily sustain analytical plasmas in alternative gases (He, N.sub.2). The use of support gases other than Ar results in simplified background spectra, which should enable the determination of Ca.sup.+, Fe.sup.+, and Se.sup.+ using their major isotopes. Argon atomic and molecular ions listed in Table 1 overlap with these analyte ions. A helium plasma should permit determination of the Halogens (F, Cl, Br, I) and non-metals (P, B, Se, Te, As) as positive ions as a result of the higher ionization potential of He Satzger, et al., 1987. In addition to a lower capital investment, reduced gas and power consumption with the MIP result in lower operating costs. Reduced demands for heat dissipation with the smaller MIP (when compared with the ICP) decrease the rate of sampling orifice deterioration.
Recent work in these laboratories has been directed toward improving the MIP as an ion source for mass spectrometry. Two major problems were encountered when using the MIP as an ion source. The first problem was that in the tangential flow torches investigated, the tangential flow was generated by forcing gas between the inside wall of a quartz tube which contains the plasma and the outer diameter of a threaded insert. Since the threaded insert which was usually machined did not perfectly match the asymmetric quartz containment tube, an asymmetric plasma was frequently generated. As the quartz tube aged, its surface became etched and eventually required replacement. Due to the irregularity of the quartz tubing, no two containment tubes would produce the same flows or the same plasma symmetry. This resulted in a change of experimental parameters with each containment tube.
The second problem was that the two dimensional spatial distribution of analyte in a MIP sustained in a cylindrical TM.sub.010 resonant cavity illustrated two optima. Investigators have found that analyte introduced into the plasma rapidly diffuses toward the periphery of the plasma and does not penetrate the region of greatest electron density Matousek, et al., 1989. Therefore, analyte ions extracted from the MIP by the vacuum system must be sampled from the periphery of the plasma where atmospheric gases are entrained Satzger, et al., 1988. This results in high levels of analyte oxides, hydroxides, nitrides, and other molecular ions in the background spectra which greatly increase the number of spectral interferences.
Major limitations to the use of microwave plasma as an ion reservoir for mass spectrometric detection have been the difficulties in controlling the plasma shape or location of the plasma in the discharge tube, and sample introduction into the microwave plasma. Although the MIP offers tremendous potential, most investigators agree that successful use of the MIP requires a great deal of dedication and patience.
The microwave plasma is a versatile source. A modified Beenakker-type TM.sub.010 resonant cavity allows plasmas to be sustained in a number of gases including Ar, He, N2 and air. However, problems with instability of the plasma discharge have led to numerous torch designs. Several investigators have developed or improved tangential flow devices which control the position of the plasma in the discharge tube by high velocity gas flows and thereby reduced degradation of the torch. See, example, Haas, D.L.; Caruso, J.A. Anal. Chem. 1984, 56, 2014; Bollo-Kamara, A.; Codding, E.G. Spectrochim, Acta 1981, 36B, 973; and Michiewicz, K.G.; Urh, J.J.; Carnahan, J.W. Spectrochim, Acta 1985. One problem with these devices was a tangential flow which was generated by gas flowing between the inside walls of a plasma containment tube and the outer diameter of a threaded insert. The symmetry of the gas flow generated by gas flowing through these threads was dependent on the fit of the threaded insert in the containment tube. Since quartz tubing is rarely perfectly round, there will be asymmetry in the flows produced and therefore in the plasma generated. Also, the tolerances of quartz tubing are not sufficiently narrow to permit replacement of the quartz containment tube while maintaining analytical performance. This lack of a good match between insert and containment tube results in the use of elevated tangential gas flow rates, which in the case of a He plasma, can greatly increase the cost of the technique.
Additional experimental difficulties in using the MIP technique arise when the spatial distribution properties of analyte in the plasma are considered. Investigators have found that analyte introduced into the plasma rapidly diffuses toward the periphery of an inhomogeneous microwave field and does not penetrate the center of the plasma (Matousek, J.P.; Orr, B.J.; Selby M. Appl. Spectrosc. 1984, 38, 231; and Selby, J.; Rezaalyaan, R.; Hieftje, G.M. Appl. Spectrosc. 1987, 41, 7497). Therefore, when the MIP technique is used as an ion source for atomic mass spectrometry, the optimum (greatest net count rate) ion sampling position corresponds to a region in which atmospheric gases are entrained in the plasma (Satzger, R.D.; Fricke, F.L.; Caruso, J.A. J. Anal. Atom. Spectrem. 1988, 3, 319). As a result, high levels of analyte oxides, hydroxides, nitrides and other molecular ions are produced which increase the complexity of the background spectra.